Runners

Source code: Lib/asyncio/runners.py

This section outlines high-level asyncio primitives to run asyncio code.

They are built on top of an event loop with the aim to simplify async code usage for common wide-spread scenarios.

Running an asyncio Program

asyncio.run(coro, *, debug=None)

Execute the coroutine coro and return the result.

This function runs the passed coroutine, taking care of managing the asyncio event loop, finalizing asynchronous generators, and closing the threadpool.

This function cannot be called when another asyncio event loop is running in the same thread.

If debug is True, the event loop will be run in debug mode. False disables debug mode explicitly. None is used to respect the global Debug Mode settings.

This function always creates a new event loop and closes it at the end. It should be used as a main entry point for asyncio programs, and should ideally only be called once.

Example:

async def main():
    await asyncio.sleep(1)
    print('hello')

asyncio.run(main())

New in version 3.7.

Changed in version 3.9: Updated to use loop.shutdown_default_executor().

Changed in version 3.10: debug is None by default to respect the global debug mode settings.

Runner context manager

class asyncio.Runner(*, debug=None, loop_factory=None)

A context manager that simplifies multiple async function calls in the same context.

Sometimes several top-level async functions should be called in the same event loop and contextvars.Context.

If debug is True, the event loop will be run in debug mode. False disables debug mode explicitly. None is used to respect the global Debug Mode settings.

loop_factory could be used for overriding the loop creation. It is the responsibility of the loop_factory to set the created loop as the current one. By default asyncio.new_event_loop() is used and set as current event loop with asyncio.set_event_loop() if loop_factory is None.

Basically, asyncio.run() example can be rewritten with the runner usage:

async def main():
    await asyncio.sleep(1)
    print('hello')

with asyncio.Runner() as runner:
    runner.run(main())

New in version 3.11.

run(coro, *, context=None)

Run a coroutine coro in the embedded loop.

Return the coroutine’s result or raise its exception.

An optional keyword-only context argument allows specifying a custom contextvars.Context for the coro to run in. The runner’s default context is used if None.

This function cannot be called when another asyncio event loop is running in the same thread.

close()

Close the runner.

Finalize asynchronous generators, shutdown default executor, close the event loop and release embedded contextvars.Context.

get_loop()

Return the event loop associated with the runner instance.

Note

Runner uses the lazy initialization strategy, its constructor doesn’t initialize underlying low-level structures.

Embedded loop and context are created at the with body entering or the first call of run() or get_loop().

Handling Keyboard Interruption

New in version 3.11.

When signal.SIGINT is raised by Ctrl-C, KeyboardInterrupt exception is raised in the main thread by default. However this doesn’t work with asyncio because it can interrupt asyncio internals and can hang the program from exiting.

To mitigate this issue, asyncio handles signal.SIGINT as follows:

  1. asyncio.Runner.run() installs a custom signal.SIGINT handler before any user code is executed and removes it when exiting from the function.

  2. The Runner creates the main task for the passed coroutine for its execution.

  3. When signal.SIGINT is raised by Ctrl-C, the custom signal handler cancels the main task by calling asyncio.Task.cancel() which raises asyncio.CancelledError inside the main task. This causes the Python stack to unwind, try/except and try/finally blocks can be used for resource cleanup. After the main task is cancelled, asyncio.Runner.run() raises KeyboardInterrupt.

  4. A user could write a tight loop which cannot be interrupted by asyncio.Task.cancel(), in which case the second following Ctrl-C immediately raises the KeyboardInterrupt without cancelling the main task.